1. Field of the Invention
This disclosure pertains to a magnetic section of a mass spectrometer, in particular, devices and methods for controlling a magnetic flux within the magnetic section.
2. Description of the Related Art
Mass spectrometry is widely used in many applications ranging from process monitoring to life sciences. Over the course of the last 60 years, a wide variety of instruments have been developed. The focus of new developments has been two fold: (1) a mass spectrometer (MS) having a high mass range and a high mass resolution, and (2) a small, desktop MS instrument.
MSs are often coupled with gas chromatographs (GC) into a gas chromatograph/mass spectrometer instrument (GC/MS) for analysis of complex mixtures, for example volatile compounds (VOCS) and semi-volatile compounds (semi-VOCs). A GC/MS instrument typically includes a gas inlet system, an electron impact based ionizer (EI) with ion extractor, optic elements to focus an ion beam, ion separation, and ion detection. Ionization can also be carried out via chemical ionization.
Ion separation can be performed in the time or spatial domain. An example of ion separation in the time domain is a time of flight mass spectrometer. Time domain ion separation is the method of ion separation commonly used in quadrupole MSs. One common type of a quadrupole mass filter allows only one mass/charge ratio to be transmitted from the ionizer to the detector. A full mass spectrum is recorded by scanning the mass range through a mass filter. Another time domain ion separation method is based on a magnetic fields where either the ion energy or the magnetic field strength is varied. Again, the mass filter allows only one mass/charge ratio to be transmitted.
Ion separation in the spatial domain is accomplished by spatially separating the ions in a magnetic field and detecting a position of the ions when they impact a position sensitive detector.
One type of MS is a double focusing MS introduced by Mattauch and Herzog (MH) in 1940 (Mattauch J., Herzog R., Über einen neuen Massenspektographen. Z. Physik, 89: (1934) 786–795. This type of MS is commonly referred to as a Mattauch-Herzog MS.
Double focusing refers to the MS's ability to refocus both an energy spread and a spatial beam spread. Modern developments in magnet and micro machining technologies allow dramatic reductions in the size of the MSs. Thus, the length of a focal plane in a modern-developed MS is reduced to a few centimeters.
The typical specifications of a small confocal plane layout Mattauch-Herzog instrument are summarized below:                Electron impact ionization, Rhenium filament        DC-voltages and permanent magnet        Ionization Energy: 0.5–2.5 kV DC        Mass Range: 2–200 D        Faraday cup detector array or strip charge detector        Duty Cycle: >99%        Read-Out time: 0.001 sec to 10 sec        
The ion optic elements can be mounted in the vacuum chamber floor on a vacuum chamber wall. In small instruments, the ion optic elements can be located on a base plate, which acts as an “optical bench” and holds all of the ion optic elements. The base plate is mounted against a vacuum flange to provide the vacuum seal needed to operate the MS under vacuum. The base plate can also be the vacuum flange itself.
The ion detector in a Mattauch-Herzog MS is a position sensitive detector. Recent developments focus on solid state based direct ion detection as an alternative to previously used electro-optical ion detectors (EOIDs).
The EOID converts the ions in a multi-channel-plate into electrons, amplifies the electrons, and illuminates a phosphorus film with the electrons. The image formed on the phosphorus film is recorded with a photo diode array via a fiber optic coupler as described in U.S. Pat. No. 5,801,380. The EOID can perform a simultaneous measurement of ions spatially separated along the focal plane of the MS. In addition, the electrons may be further converted to photons that form images of the ion-induced signals.
The ions generate electrons by impinging on a microchannel electron multiplier array. The electrons are accelerated to a phosphor-coated fiber-optic plate that generates photon images using a photodetector array. Some drawbacks of the EOID is that it requires multiple conversions. In addition, the use of phosphors may limit the dynamic range of the EOID. The microchannel device may require a high-voltage, for example 1 KV, which could require that the microchannel device be placed in the vacuum chamber under a pressure of about 106 Torr. Under such pressure, the microchannel device may experience ion feedback, electric discharge, and fringe magnetic fields may affect the electron trajectory. Further, a resolution of the EOID may be adversely affected by isotropic phosphorescence emission, which may also affect the resolution of the mass analyzer.
Another type of detector is a micro-machined Faraday cup detector, which comprises an array of individually addressable Faraday cups for monitoring the ion beam. One type of Faraday cup detector is described in detail in “A. A. Scheidemann, R. B. Darling, F. J. Schumacher, and A. Isakarov, Tech. Digest of the 14th Int. Forum on Process Analytical Chem. (IFPAC-2000), Lake Las Vegas, Nev., Jan. 23–26, 2000, abstract 1–067”; “R. B. Darling, A. A. Scheidemann, K. N. Bhat, and T. C. Chen, Proc. of the 14th IEEE Int. Conf. on Micro Electro Mechanical Systems (MEMS 2001), Interlaken, Switzerland, Jan. 21–25, 2001, pp. 90–93”; and U.S. patent application Ser. No. 09/744,360.
Other references of interest are “Nier, D. J. Schlutter Rev. Sci. Instrum. 56(2), page 214–219, 1985”; and “T. W. Burgoyne et. al. J. Am. Soc. Mass Spectrum 8, pages 307–318, 1997.”
Another type of detector that can be used for low energy ions is a flat metallic strip, called a strip charge detector (SCD).
Yet another type of detector is a shift register based direct ion detector, which is described in U.S. Pat. No. 6,576,899.
The shift register based direction detector may be used in a GC/MS instrument. The shift register based direct ion detector allows direct measurement of ions in a MS without conversion to electrons and photons (e.g., EOID) prior to measurement. The shift register based direct ion detector may incorporate charge coupled device (CCD) technology, which includes the use of metal oxide semiconductors. Detected charged particles form a signal charge that directly accumulates in a shift register associated with a part of the CCD. The signal charge can be clocked through the CCD to a single output amplifier. Since the CCD uses only one charge-to-voltage conversion amplifier, signal gains and offset variations of individual elements in the detector array can be minimized.
In the Mattauch-Herzog MS, the detector, which can be a Faraday cup detector, a strip charge detector, or one of the aforementioned detectors, is placed at an exit of a magnet section called a focal plane.
In a Mattauch-Herzog MS, the ion optic elements are placed on the vacuum chamber wall and the ion detector is mounted on an exit flange of the ion flight path. This arrangement is required as a result of having the magnet section outside of the vacuum chamber. A multiplexer and an amplifier are also positioned outside of the vacuum chamber in the Mattauch-Herzog MS.
FIG. 1 shows a Mattauch-Herzog double focusing MS 10 assembled with a GC 12, the MS 10 includes an ionizer 14, a shunt and aperture 16, an electro static energy analyzer 18, a magnetic section 20, and a focal plane section 22.
In the operation of the MS 10, a gaseous material or a vapor is introduced into the ionizer 14, either directly or through the GC 12 (for complex mixtures or compounds). The material is bombarded by electrons to produce ions. The ions are focused in the shunt and aperture section 16 to form an ion beam 24. The ions are separated according to their charge/mass ratio as they move through the electro static energy analyzer 18 and the magnetic section 20. The ions are then detected in the focal plane section 22, as described in U.S. Pat. No. 5,801,380. The ion separation process takes place under a vacuum pressure on the order of about 10−5 Torr, which can be achieved with a vacuum pump (not shown).
The GC 12 includes a sample injector valve V, which has an entry port S for introduction of the sample, an exit port W for the waste after the sample has been vaporized and/or decomposed, typically by heat. The sample injector valve V may be a liquid injector. The part to be analyzed, referred to as analyte is carried by a carrier gas, such as dry air, hydrogen, or helium, for example, to a capillary microbore column M (wall coated open tubular, or porous layer open tubular, or packed, etc.), where its constituents are separated by different absorption rates on the wall of the microbore column M. The microbore column M has a rather small inside diameter, of the order of about 50–500 μm in the illustrated embodiment. The carrier gas flow rate is about 0.2 to 5 atm. cm3/sec, although higher flow rates, for example 20 atm. cm3/sec, are possible.
A larger microbore column M bore requires a larger vacuum pump, whereas a smaller bore produces narrower peaks of the effluent, which may result in a loss of signal. In general, the gas flow rate is a function of the inner diameter, the length of the column M, the pressure of the carrier gas, and the temperature of the carrier gas. The width of the peak again is a function of the injection time, the stationary phase of the column (e.g., polarity, film thickness, distribution in the column), the width and length of the column, the temperature and the gas velocity. One method of determining a size of the microbore column M bore is addressed in U.S. Pat. No. 6,046,451.
Temperature variations, especially in the magnetic section of the MS, induce the need for frequent calibrations of the MS. The temperature variations can be caused by environmental factors and/or from internal components, such as the ionizer, which utilizes a hot electrode (e.g., a glowing rhenium filament). Frequent calibrations are time consuming and operators tend to avoid them, which eventually leads to inaccuracy in the measurements.
Permanent magnets located in the magnetic section may be especially susceptible to the temperature variations. It is understood that the volume and the mass of a permanent magnet is typically inversely proportional to an energy product value of the magnet. One type of magnetic material is AINiCo V, which has an energy product of about 5–6 MGOe. Other magnet materials include, but are not limited to steel, Sm—Co alloys and Nd—B—Fe alloys. These materials are sensitive to temperature variations. Methods for temperature compensation to avoid frequent instrument calibrations and/or other issues are described in U.S. Pat. No. 6,403,956 and in U.S. Provisional Patent Application No. 60/557,920.
Other patents of interest are U.S. Pat. No. 5,317,151; U.S. Pat. No. 6,182,831; and U.S. Pat. No. 6,191,419.